what does npm -D flag mean?
Solution 1:
The -D flag is the shortcut for: --save-dev. Source: https://docs.npmjs.com/cli/install
-D, --save-dev: Package will appear in your devDependencies.
Solution 2:
As described in the NPM Install Docs:
-D,
--save-dev: Package will appear in yourdevDependencies.
Which means that the package will not be installed if you do npm install --production.
A detailed explanation of the different types of dependencies: SO-Answer